The present invention relates to a process for the preparation of catalysts based on MFI-type zeolite prepared in spheroidal form and active in transposition reactions of oximes to amides.
More specifically, the present invention relates to a process for the preparation of catalysts based on MFI-type zeolites, suitable for use in gas phase in fluid-bed and entrained-bed reactors.
The invention also relates to catalysts obtained by the above processes.
Zeolites of the MFI type, in particular those with a high silica/alumina ratio (U.S. Pat. No. 4,359,421) and, more generally, those with a low content of trivalent heteroelements (patent EP 242.960), are known in literature as basic material for the preparation of catalysts which can be used in the transposition reaction of oximes to amides.
Among these reactions, those carried out in gas phase which use solids based on zeolites are of particular importance. For example, the patent EP 234.088 describes a method for the preparation of xcex5-caprolactam which consists in putting cyclohexanonoxime, in gaseous state, in contact with crystalline alumino-silicates having well-defined physico-chemical characteristics and preformed in the form of granules (24-48 mesh).
Although they consist of active component alone, these materials have limited possibilities of being used in industrial reactors; in fact, if fluid-bed or entrained-bed reactors are used for the catalytic process, the catalysts should preferably be in the form of microspheres, with an average diameter of 30-100 xcexcm and characterized by a high resistance to interparticle attrition and attrition against the walls of the reactors; if, on the other hand, fixed-bed reactors are used, the catalysts should have the typical forms for this technology (speres, tablets, etc.) with dimensions of approximately a few millimeters and characterized by a good mechanical stress resistance.
The cohesion between the individual and submicronic particles of the microcrystalline zeolitic material is generally poor and consequently resistance to attrition and mechanical stress is normally obtained by combining the zeolitic material with compounds of an inorganic nature (ligands) in forming phase.
Catalysts based on zeolites, suitable for use in fluid-bed or entrained-bed reactors and with the characteristics listed above, are widely described in the known art and are mainly used in catalytic cracking processes (FCC catalysts, Fluid Catalytic Cracking).
The forming of the above catalysts, when microspheres with a diameter  less than 100 xcexcm are required, is usually carried out with the well-known spray-drying technique.
Silicas and aluminum oxides in the colloidal state or silico-aluminates are used as ligands to give the microspheres a higher mechanical resistance to attrition; the quantities of the above ligands in the resulting catalytic compositions is normally less than 30% by weight.
The use of siliceous ligands in spherulization processes of MFI-type zeolitic materials suitable for the transposition reaction of oximes to amides, however, is not widely used as amorphous silica is not completely inactive in the above reactions.
It is known in fact that this compound may significantly jeopardize the selectivity of the zeolitic catalyst and deteriorate the catalytic performance due to the formation of organic pitches.
It has been observed, for example, that silica gels characterized by the presence of hydroxylic groups with a low acidity, have a high activity, low selectivity and rapid formation of pitch in transposition reactions of cyclohexanonoxime to xcex5-caprolactam [Catalysis Letters 17 (1993), 139-140; Catalysis Today 38 (1997), 249-253].
To overcome this problem patent EP 576.295 suggests, for example, preforming the zeolitic material in a spherical form by spray-drying without any addition of ligands and subjecting the microspheres, in a subsequnent process phase, to thermal treatment in water to increase their hardness.
A process has now been found for the preparation of catalysts based on zeolitic materials and siliceous ligands, suitable for use in reactions of oximes to amides, which overcomes the drawbacks of the known art.
In particular, the present invention relates to a process for the preparation of catalysts based on MFI-type zeolitic compounds and siliceous ligands characterized in that submicronic particles of zeolite are incorporated into a ligand obtained by the acid hydrolysis of silicon alkoxide.
The innovative aspect of the invention basically consists in the use of oligomeric species of silica synthesized in an acid medium as ligands of MFI-type zeolitic materials.
The catalysts obtained by the process of the invention have a high resistance to attrition and mechanical stress and an excellent catalytic activity.
In particular, the siliceous ligands used, consisting of oligomeric species of silica synthesized by the acid hydrolysis of silicon alkoxides, overcome the disadvantages deriving from their use in the transposition reaction of oximes to amides, because they do not interact in the reaction itself.
This behaviour is in distinct contrast with that of catalysts containing ligands prepared in an alkaline medium, as described, for example, in U.S. Pat. No. 4,859,785 (hydrolysis of silicon alkoxides with organic bases), or basic solutions of colloidal silicons, known commercially under the trade-name of xe2x80x9cLudoxxe2x80x9d.
It has also been found that the ligands used in the process of the invention can also be advantageously used in the known forming operations of zeolitic materials into spherical form.
In particular, the ligands of the invention have been used in emulsifying/gelation techniques, which consist in dispersing the submicronic particles of zeolitic material in the acid solution of the siliceous ligand and subjecting the resulting mixture to emulsification and gelation in an organic medium.
With this technique, catalytic compositions are obtained in spherical form and with morphological-granulometric, physico-chemical characteristics and resistance to attrition suitable for use in the transposition reactions of oximes to amides in fluid-bed or entrained-bed reactors.
In particular, the catalytic compositions based on MFI-type zeolite which are obtained, consist of microspheres with an average diameter varying from 20 to 200 xcexcm and are characterized by a content of siliceous ligand (expressed as SiO2) equal to or higher than 30% by weight, an external surface area (A.S.E.) varying from 60 to 80 m2/g, a specific volume of the micro and mesopores varying from 0.2 to 0.3 ml/g and 0.1 to 0.4 ml/g respectively, bulk density values ranging from 0.7 to 0.85 g/ml and resistance to attrition, expressed as D.I. (Davison Index) less than or equal to 6.
The above catalysts significantly differ from those known in the art in their high content of siliceous ligand (xe2x89xa730%), thus confirming the low catalytic activity of the ligand in transposition reactions of oximes to amides.
A further object of the invention consists of a process for the catalytic transposition of oximes to amides characterized in that the reaction is carried out in the presence of the catalysts of the invention.
In particular, the catalysts obtained by the emulsification/gelation technique have morphological-granulometric, physico-chemical characteristics and resistance to attrition suitable for use in transposition reactions of oximes to amides in fluid-bed or entrained-bed reactors.
The emulsification/gelation technique used for forming the catalysts of the present invention comprises in particular the following steps:
preparation of siliceous oligomers by the hydrolysis of silicon alkoxides;
mixing said oligomers with an aqueous or hydroalcohol dispersion of submicronic particles of MFI-type zeolite acidified to pHxe2x89xa65;
spherulization of the resulting mixture.
In the preparation of the siliceous oligomers, silicon alkoxides are used as silica precursors, such as for example tetra-ethyl-orthosilicate (TEOS). The hydrolysis of these compounds in an aqueous medium catalyzed by acids, as well as the effect of the hydrolysis conditions on the physico-chemical characteristics of the siliceous oligomers, are widely described in the art [C. J. Brinker, G. W. Shere xe2x80x9cSol-Gel Science. The Physic and Chemistry of sol-gel processingxe2x80x9d, Academic Press Inc., 1990].
Siliceous oligomers suitable for the purposes of the present invention are preferably obtained by the hydrolysis of TEOS in an aqueous medium and in the presence of mineral acids, such as for example, HCl and HNO3, regulating the molar ratio H2O/TEOS between 10 and 25 and the pH between 1.5 and 3.0. The hydrolysis reaction is carried out maintaining the reagents (TEOS and acid aqueous solution) under mechanical stirring for times varying from 1 to 3 hours at temperatures generally between 20 and 40xc2x0 C. The concentration of alcohol in the reaction mixture (in particular of ethanol deriving from the hydrolysis of TEOS) may be appropriately corrected in a subsequent operation depending on the technology adopted in the spherulization of the zeolitic material. For example the solution of siliceous oligomers may be dealcoholated and concentrated by distillation at reduced pressure and at a temperature of less than 30xc2x0 C., or it may be further diluted with alcohol or with hydro-alcohol solutions.
The MFI-type zeolitic compounds suitable for the transposition reaction of oximes to amides can be selected from Silicalite-1 or zeolites with a low content of aluminum (molar ratio Si/Al greater than 1000) or other trivalent or tetravalent heteroatoms, such as those of Group III (B, Ga, In) or Ti.
As described in the known art, these materials are obtained by hydrothermal synthesis from a mixture of reagents comprising a precursor of silica with a high purity (for example TEOS), water, alcohols, organic amines or cations of tetra-alkyl-ammonium (RnN+) as agents for controlling the crystallization (templating agents) of the zeolitic material.
The reaction product, consisting of individual microcrystalline particles, normally having dimensions of less than 1 xcexcm, is generally separated from the mother liquor by centrifugation, is then repeatedly washed with water to remove the excess templating agent and finally dried and calcined. Alternatively the reaction product can be spray-dried.
In the process for the prepration of the catalysts of the present invention the zeolitic intermediate is advantageously centrifuged and washed with water. This intermediate, in a thickened form, is dispersed in an aqueous or hydro-alcohol medium using mechanical dispersers or also ultrasonic devices and the dispersion conditions are checked to ensure that the dimensions of the material reach values close to those of the individual particles (normally less than 1 xcexcm). The control of the dispersion degree of the zeolitic material in the aqueous medium is particularly important when dried zeolitic intermediates, and mainly zeolitic materials subjected to thermal treatment at a temperature of xe2x89xa7500xc2x0 C., are used in the process.
Under the preferred conditions, when a zeolitic intermediate which has been centrifuged and optionally washed, is used, the pH of the resulting aqueous or hydro-alcohol solution is normally alkaline owing to the incomplete removal of the templating agent. To avoid the formation of undesired polymerization or gelation phenomena of the acid siliceous oligomers in the subsequent mixing operation, the above dispersions are acidified up to pH values less than or equal to 5.0. This operation, which does not usually appear in the description of processes for the preparation of zeolitic materials, is therefore a fundamental point of the present invention.
The acidification can be carried out with solutions of mineral or organic acids and, under the preferred conditions, with the type of acid used in the preparation of the siliceous ligand, such as for example HCl and HNO3. The quantity of acid is controlled so that the pH of the resulting ligand/zeolite mixture is less than 4.0, preferably between 2.0 and 3.0 inclusive.
With respect to the composition of the above mixture, the weight ratio between the MFI-type zeolitic compound and the siliceous ligand (expressed as SiO2) can be extended up to values of 2.5 inclusive, thus obtaining catalytic compositions in which the siliceous ligand is contained in a quantity of up to 50% by weight; under the preferred conditions the siliceous ligand is between 30 and 50% by weight.
The concentration of the solid (MFI-type zeolite) in the aqueous or hydro-alcohol solution of the siliceous ligand is established each time in relation to the technology adopted to obtain materials in spherical form, such as for example, spray-drying or emulsification/gelation in organic media.
In particular the spherulization process followed to illustrate the invention consists in emulsifying the aqueous or hydro-alcohol ligand/zeolite mixture in an organic medium (decanol) and consolidating the microdrops of dispersed phase by rapidly bringing the emulsion in contact with a basic solution (solution of cyclohexylamine in decanol). The microspheres of gel thus obtained are subsequently separated from the basic solution, washed repeatedly with ethanol, dried and finally calcined in an oxidating atmosphere (air) at a temperature of over 450xc2x0 C. and, normally, within the typical temperature range of zeolitic materials (500-550xc2x0 C.) with a heating rate generally of about 50xc2x0 C./h.
The materials prepared with the above process consist of microspheres whose dimensions can vary from 20 to 200 xcexcm depending on the emulsifying conditions of the ligand/zeolite mixture. These materials, owing to their morphological-granulometric and physico-chemical characteristics described above, can be conveniently used in processes for the preparation in gas phase of amides by the catalytic transposition of oximes.
Among amides, which form an important group of intermediates, caprolactam is considerably important, in particular for the preparation of polyamidic resins and synthetic fibres.
This compound is at present produced industrially by transposition in liquid phase with sulfuric acid or oleum, a technology that has numerous problems linked to the use of sulfuric acid.
The necessity for an alternative process for the preparation of caprolactam such as that of the present invention was therefore particularly felt.
In particular, the catalysts of the present invention can be advantageously used in the transposition reaction of cyclohexanonoxime to xcex5-caprolactam with a process in gas phase, consisting in bringing cyclohexanonoxime vapours in contact with the catalyst.
This reaction can be carried out at a pressure ranging from 0.05 to 10 bars and at a temperature ranging from 250 to 500xc2x0 C., preferably from 300 to 450xc2x0 C.
More specifically, the cyclohexanonoxime is fed to a reactor containing the catalyst, in vapour phase and in the presence of one or more solvents and, optionally, also an uncondensable gas. Under the preferred conditions the cyclohexanonoxime is dissolved in a mixture of solvents, described below, at a concentration ranging from 5 to 25% by weight and preferably between 6 and 15%; the solution thus obtained is then vaporized and fed to the reactor.
Preferred solvents are of the type R1xe2x80x94Oxe2x80x94R2 wherein R1 is a C1-C4 alkyl chain and R2 can be a hydrogen atom or an alkyl chain containing a number of carbon atoms less than or equal to R1. Alcohols with a C1-C2 alkyl chain are particularly preferred. These solvents can be used alone or mixed with each other, or combined with an aromatic hydrocarbon such as benzene or toluene.
The feeding rate of the cyclohexanonoxime is checked so that the WHSV value (Weight Hourly Space Velocity), expressed as kg of cyclohexanonoxime/kg of catalyst/h, is between 0.1 and 50 hxe2x88x921, preferably between 0.5 and 20 hxe2x88x921.
During the transposition reaction of cyclohexanonoxime to xcex5-caprolactam, the catalytic performance undergoes deterioration due to the formation of organic pitches which obstruct the pores of the catalyst and poison its active centres. The deterioration process is slow and depends on the operating conditions and, in particular, the WHSV, type of solvents, feeding composition, temperature and pressure. The catalytic activity however can be effectively re-integrated by combustion of the pitches, oxidating treatment in a stream of air, optionally diluted with nitrogen, at a temperature ranging from 450 to 550xc2x0 C., preferably between 450 and 500xc2x0 C.
The presence of silica in the catalytic composition does not substantially jeopardize the catalytic performance of the zeolitic material (active component). In fact, if the reaction is carried out under identical WHSV conditions (referring to the weight of active component in the catalyst), the conversion yield of cyclohexanonoxime and, above all, the selectivity to xcex5-caprolactam are in line with those of the catalyst consisting of active component alone. Also the pitching rate does not undergo variations; the deterioration in the performance therefore remains unaltered and it is not necessary to increase the frequency of the regenerations.